Low-temperature high-resolution Fourier-transform infra-red stretching-mode absorption and dielectric susceptibility measurements of OH- and OD- defects in these crystals were described. Although very low concentrations were used, it was always found that there were significant deviations from the behavior expected for isolated tunnelling defects. A major result was that the tunnelling of hydroxide and deuteroxide defects in both materials exhibited an unusual isotopic dependence: the level splitting associated with the tunnelling motion of the heavier defect ion, OD-, was greater than that due to OH- defects.

Anomalous Isotope Dependence of Tunnel Splitting of OH- and OD- Defects in KCl and NaCl Crystals. S.Ludwig, A.Brederek, C.Enss, C.P.An, F.Luty: Physical Review Letters, 2000, 85[26], 5591-4